FIG. 23 is a block diagram for representing an arrangement of a conventional dimming (light controlling) apparatus of a fluorescent lamp, similar to the fluorescent lamp dimming apparatus described in, for example, Japanese Patent Application Laid-Open No. Hei 6-333692.
In FIG. 23, reference numeral 1 indicates a high frequency power supply, reference numeral 2 indicates a coil, reference numeral 3 shows a fluorescent lamp (will be simply referred to as a lamp hereinafter), reference numeral 4 denotes a control unit of the high frequency power supply 1, and reference numeral 5 represents a dimming (light controlling) signal.
In the apparatus shown in FIG. 23, the control unit 4 controls the frequency of the high frequency power supply 1 in response to the dimming signal 5 entered thereinto. Since the high frequency power supply 1 is connected via the coil 2 to the lamp 3, the impedance of the coil 2 is varied by the frequency of the high frequency power supply 1. In connection with this impedance change, a high frequency current flowing through the lamp 3 is varied. In other words, the higher the frequency of the high frequency power supply 1 becomes, the smaller the current flowing through the lamp 3 becomes, so that the lamp 3 is brought into dimming states.
Since the dimming degree of the lamp 3 is substantially directly in proportion to the current flowing through the lamp 3, the control unit 4 controls the frequency of the high frequency power supply 1 in response to the inputted dimming signal 5 so as to dim the lamp 3. For the sake of convenience, in the case that the dimming signal 5 indicates a specific dimming degree, a frequency outputted from the high frequency power supply 1 is referred to as a dimming frequency, and also, a dimming degree at this time is referred to as a set dimming degree (% indication).
FIG. 24 is a graphic representation for showing an example of the output frequency of the high frequency power supply 1 with respect to the set dimming degree. The control unit 4 controls the high frequency power supply 1 in such a way that when the set dimming degree is equal to, for example, 100%, the frequency becomes 50 kHz, whereas when the set dimming degree is equal to 25%, the frequency becomes 80 kHz.
The light output from the lamp 3 under this control condition is indicated in a graphic representation of FIG. 25. In FIG. 25, 0.degree. C., 10.degree. C., and 25.degree. C. represent atmospheric temperatures of the lamp 3. Even under the same set dimming degree, the light outputs are different from each other, depending upon the lamp atmospheres. This may be caused by the characteristics of the lamp 3, namely the lamp impedance of the lamp 3 owns the temperature characteristic. For instance, when the set dimming degree is equal to 100%, the light output becomes L1 at 25.degree. C.; the light output becomes L2 at 10.degree. C.; the light output becomes L3 at 0.degree. C., and thus, a relationship of L1&gt;L2&gt;L3 is established.
Also, in such a case that the atmospheric temperature of the lamp 3 is 25.degree. C., the light output is continuously changed with respect to the set dimming degree. To the contrary, when the atmospheric temperatures of the lamp 3 are equal to 10.degree. C. and 0.degree. C., if the set dimming degree is decreased, then the light outputs are rapidly changed, so that discontinuous points appear.
As apparent from the above graphic representation, when the atmospheric temperature of the lamp 3 is 0.degree. C., the light output is changed from a point A to a point B in the vicinity of the set dimming degree of 40% (between 35% and 45%), and the light output becomes very small at the point B. Also, when the atmospheric temperature of the lamp 3 is 10.degree. C., a similar phenomenon occurs in the vicinity of the set dimming degree of 30%.
This reason is given as follows. When the atmospheric temperature of the lamp 3 is low (less than or equal to 10.degree. C.), the lamp voltage is increased in the dimming degree lower than, or equal to a certain dimming degree, as compared with that of the normal temperature. Also, the lower the dimming degree becomes, the stronger the increasing trend of the lamp voltage is changed. When the lamp voltage is rapidly increased, the operating point of the current flowing from the high frequency power supply 1 to the coil 2 and the lamp 3 becomes unstable. The lamp current is suddenly decreased by feeding such a loop that the current flowing through the lamp 3 is decreased.fwdarw.the impedance of the lamp 3 is increase.fwdarw.the current flowing through the lamp 3 is decreased.
At this time, there are some possibilities that the light output is fluctuated, depending upon the condition of the lamp 3. Since the lamp voltage is low in the normal temperature (25.degree. C.), the operating point becomes one, and the lamp current may flow under stable condition by the coil 2.
Since the above-described conventional fluorescent lamp dimming apparatus is arranged in the above-described manner, when the lamp peripheral temperature is low, the light output is rapidly lowered, depending upon both the temperature characteristics of the lamp 3 and the circuit condition from the high frequency power supply 1. As a result, there are such problems that the lamp cannot be dimmed in the continuous manner, and also the light output is fluctuated.
The present invention has been made to solve these problems, and has an object to provide a fluorescent lamp dimming apparatus capable of continuously dimming the fluorescent lamp, and also capable of eliminating fluctuations of light outputs.